Add suggested changes

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -116,6 +116,7 @@ object SpaceEngine {
   def isSubspace(a: Space, b: Space)(using Context): Boolean = a.isSubspace(b)
   def canDecompose(typ: Typ)(using Context): Boolean         = typ.canDecompose
   def decompose(typ: Typ)(using Context): List[Typ]          = typ.decompose
+  def nullSpace(using Context): Space = Typ(ConstantType(Constant(null)), decomposed = false)
 
   /** Simplify space such that a space equal to `Empty` becomes `Empty` */
   def computeSimplify(space: Space)(using Context): Space = trace(i"simplify($space)")(space match {
@@ -336,6 +337,13 @@ object SpaceEngine {
     case pat: Ident if isBackquoted(pat) =>
       Typ(pat.tpe, decomposed = false)
 
+    case Ident(nme.WILDCARD) =>
+      val tp = pat.tpe.stripAnnots.widenSkolem
+      val isNullable = tp.isInstanceOf[FlexibleType] || tp.classSymbol.isNullableClass
+      val tpSpace = Typ(erase(tp, isValue = true), decomposed = false)
+      if isNullable then Or(tpSpace :: nullSpace :: Nil)
+      else tpSpace
+
     case Ident(_) | Select(_, _) =>
       Typ(erase(pat.tpe.stripAnnots.widenSkolem, isValue = true), decomposed = false)
 
@@ -667,7 +675,7 @@ object SpaceEngine {
           case tp                                                          => (tp, Nil)
         val (tp, typeArgs) = getAppliedClass(tpOriginal)
         // This function is needed to get the arguments of the types that will be applied to the class.
-        // This is necessary because if the arguments of the types contain Nothing, 
+        // This is necessary because if the arguments of the types contain Nothing,
         // then this can affect whether the class will be taken into account during the exhaustiveness check
         def getTypeArgs(parent: Symbol, child: Symbol, typeArgs: List[Type]): List[Type] =
           val superType = child.typeRef.superType
@@ -923,50 +931,48 @@ object SpaceEngine {
     && !sel.tpe.widen.isRef(defn.QuotedExprClass)
     && !sel.tpe.widen.isRef(defn.QuotedTypeClass)
 
-  def mayCoverNull(tp: Space)(using Context): Boolean = tp match
-    case Empty => false
-    case Prod(_, _, _) => false
-    case Typ(tp, decomposed) => tp == ConstantType(Constant(null)) 
-    case Or(ss) => ss.exists(mayCoverNull)
-
   def checkReachability(m: Match)(using Context): Unit = trace(i"checkReachability($m)"):
     val selTyp = toUnderlying(m.selector.tpe).dealias
     val isNullable = selTyp.isInstanceOf[FlexibleType] || selTyp.classSymbol.isNullableClass
     val targetSpace = trace(i"targetSpace($selTyp)"):
       if isNullable && !ctx.mode.is(Mode.SafeNulls)
       then project(OrType(selTyp, ConstantType(Constant(null)), soft = false))
       else project(selTyp)
-
-    @tailrec def recur(cases: List[CaseDef], prevs: List[Space], deferred: List[Tree], nullCovered: Boolean): Unit =
+    var hadNullOnly = false
+    @tailrec def recur(cases: List[CaseDef], prevs: List[Space], deferred: List[Tree]): Unit =
       cases match
         case Nil =>
-        case (c @ CaseDef(pat, guard, _)) :: rest =>
-          val patNullable = Nullables.matchesNull(c)
-          val curr = trace(i"project($pat)")(
-            if patNullable
-            then Or(List(project(pat), Typ(ConstantType(Constant(null)))))
-            else project(pat))
+        case CaseDef(pat, guard, _) :: rest =>
+          val curr = trace(i"project($pat)")(project(pat))
           val covered = trace("covered")(simplify(intersect(curr, targetSpace)))
           val prev = trace("prev")(simplify(Or(prevs)))
           if prev == Empty && covered == Empty then // defer until a case is reachable
-            recur(rest, prevs, pat :: deferred, nullCovered)
+            recur(rest, prevs, pat :: deferred)
           else
             for pat <- deferred.reverseIterator
             do report.warning(MatchCaseUnreachable(), pat.srcPos)
 
             if pat != EmptyTree // rethrow case of catch uses EmptyTree
                 && !pat.symbol.isAllOf(SyntheticCase, butNot=Method) // ExpandSAMs default cases use SyntheticCase
-                && isSubspace(covered, Or(List(prev, Typ(ConstantType(Constant(null))))))
             then
-              val nullOnly = isNullable && isWildcardArg(pat) && !nullCovered && !isSubspace(covered, prev) && (!ctx.explicitNulls || selTyp.isInstanceOf[FlexibleType])
-              if nullOnly then report.warning(MatchCaseOnlyNullWarning() , pat.srcPos)
-              else if (isSubspace(covered, prev)) then report.warning(MatchCaseUnreachable(), pat.srcPos)
+              if isSubspace(covered, prev) then
+                report.warning(MatchCaseUnreachable(), pat.srcPos)
+              else if isNullable && !hadNullOnly && isWildcardArg(pat)
+                && isSubspace(covered, Or(prev :: nullSpace :: Nil)) then
+                // Issue OnlyNull warning only if:
+                // 1. The target space is nullable;
+                // 2. OnlyNull warning has not been issued before;
+                // 3. The pattern is a wildcard pattern;
+                // 4. The pattern is not covered by the previous cases,
+                //    but covered by the previous cases with null.
+                hadNullOnly = true
+                report.warning(MatchCaseOnlyNullWarning(), pat.srcPos)
 
             // in redundancy check, take guard as false in order to soundly approximate
-            val newPrev = if (guard.isEmpty) then covered :: prevs else prevs
-            recur(rest, newPrev, Nil, nullCovered || (guard.isEmpty && patNullable))
+            val newPrev = if guard.isEmpty then covered :: prevs else prevs
+            recur(rest, newPrev, Nil)
 
-    recur(m.cases, Nil, Nil, false)
+    recur(m.cases, Nil, Nil)
   end checkReachability
 
   def checkMatch(m: Match)(using Context): Unit =

tests/explicit-nulls/warn/i21577.check

@@ -1,25 +1,29 @@
 -- [E121] Pattern Match Warning: tests/explicit-nulls/warn/i21577.scala:5:9 --------------------------------------------
-5 |    case _ => // warn
+5 |    case _ => // warn: null only
   |         ^
   |         Unreachable case except for null (if this is intentional, consider writing case null => instead).
 -- [E121] Pattern Match Warning: tests/explicit-nulls/warn/i21577.scala:12:9 -------------------------------------------
-12 |    case _ => // warn
+12 |    case _ => // warn: null only
    |         ^
    |         Unreachable case except for null (if this is intentional, consider writing case null => instead).
+-- [E121] Pattern Match Warning: tests/explicit-nulls/warn/i21577.scala:16:7 -------------------------------------------
+16 |  case _ => // warn: null only
+   |       ^
+   |       Unreachable case except for null (if this is intentional, consider writing case null => instead).
 -- [E030] Match case Unreachable Warning: tests/explicit-nulls/warn/i21577.scala:20:7 ----------------------------------
-20 |  case _ => // warn
+20 |  case _ => // warn: unreachable
    |       ^
    |       Unreachable case
 -- [E029] Pattern Match Exhaustivity Warning: tests/explicit-nulls/warn/i21577.scala:29:27 -----------------------------
-29 |def f7(s: String | Null) = s match // warn
+29 |def f7(s: String | Null) = s match // warn: not exhuastive
    |                           ^
    |                           match may not be exhaustive.
    |
    |                           It would fail on pattern case: _: Null
    |
    | longer explanation available when compiling with `-explain`
 -- [E029] Pattern Match Exhaustivity Warning: tests/explicit-nulls/warn/i21577.scala:36:33 -----------------------------
-36 |def f9(s: String | Int | Null) = s match // warn
+36 |def f9(s: String | Int | Null) = s match // warn: not exhuastive
    |                                 ^
    |                                 match may not be exhaustive.
    |

tests/explicit-nulls/warn/i21577.scala

@@ -2,22 +2,22 @@ def f(s: String) =
   val s2 = s.trim()
   s2 match
     case s3: String =>
-    case _ => // warn
+    case _ => // warn: null only
 
 
 def f2(s: String | Null) =
   val s2 = s.nn.trim()
   s2 match
     case s3: String =>
-    case _ => // warn
+    case _ => // warn: null only
 
 def f3(s: String | Null) = s match
   case s2: String =>
-  case _ =>
+  case _ => // warn: null only
 
 def f5(s: String) = s match
   case _: String =>
-  case _ => // warn
+  case _ => // warn: unreachable
 
 def f6(s: String) = s.trim() match
   case _: String =>
@@ -26,13 +26,13 @@ def f6(s: String) = s.trim() match
 def f61(s: String) = s.trim() match
   case _: String =>
 
-def f7(s: String | Null) = s match // warn
+def f7(s: String | Null) = s match // warn: not exhuastive
   case _: String =>
 
 def f8(s: String | Null) = s match
   case _: String =>
   case null =>
 
-def f9(s: String | Int | Null) = s match // warn
+def f9(s: String | Int | Null) = s match // warn: not exhuastive
   case _: String =>
   case null =>

tests/patmat/null.check

@@ -1,3 +1,3 @@
-6: Match case Unreachable
+6: Pattern Match
 13: Pattern Match
 20: Pattern Match

tests/warn/i20121.scala

@@ -5,8 +5,8 @@ case class CC_B[A](a: A) extends T_A[A, X]
 
 val v_a: T_A[X, X] = CC_B(null)
 val v_b = v_a match
-  case CC_B(_) => 0 // warn: unreachable
-  case _       => 1
+  case CC_B(_) => 0
+  case _       => 1 // warn: null only
     // for CC_B[A] to match T_A[X, X]
     // A := X
     // so require X, aka T_A[Byte, Byte]

tests/warn/i20122.scala

@@ -7,7 +7,7 @@ case class CC_E(a: CC_C[Char, Byte])
 
 val v_a: T_B[Int, CC_A] = CC_B(CC_E(CC_C(null)))
 val v_b = v_a match
-  case CC_B(CC_E(CC_C(_))) => 0 // warn: unreachable
+  case CC_B(CC_E(CC_C(_))) => 0
   case _                   => 1
     // for CC_B[A, C] to match T_B[C, CC_A]
     // C <: Int, ok

tests/warn/i20123.scala

@@ -8,7 +8,7 @@ case class CC_G[A, C](c: C) extends T_A[A, C]
 val v_a: T_A[Boolean, T_B[Boolean]] = CC_G(null)
 val v_b = v_a match {
   case CC_D()  => 0
-  case CC_G(_) => 1 // warn: unreachable
+  case CC_G(_) => 1
     // for CC_G[A, C] to match T_A[Boolean, T_B[Boolean]]
     // A := Boolean, which is ok
     // C := T_B[Boolean],

tests/warn/redundant-null.check

@@ -1,52 +1,56 @@
 -- [E030] Match case Unreachable Warning: tests/warn/redundant-null.scala:10:7 -----------------------------------------
-10 |  case _: n.type => // warn
+10 |  case _: n.type => // warn: unreachable
    |       ^^^^^^^^^
    |       Unreachable case
 -- [E030] Match case Unreachable Warning: tests/warn/redundant-null.scala:12:7 -----------------------------------------
-12 |  case _ => // warn
+12 |  case _ => // warn: unreachable
    |       ^
    |       Unreachable case
 -- [E030] Match case Unreachable Warning: tests/warn/redundant-null.scala:13:7 -----------------------------------------
-13 |  case _ => // warn
+13 |  case _ => // warn: unreachable
    |       ^
    |       Unreachable case
 -- [E030] Match case Unreachable Warning: tests/warn/redundant-null.scala:18:7 -----------------------------------------
-18 |  case _ => 3 // warn
+18 |  case _ => 3 // warn: unreachable
    |       ^
    |       Unreachable case
 -- [E030] Match case Unreachable Warning: tests/warn/redundant-null.scala:23:7 -----------------------------------------
-23 |  case _: B => // warn
+23 |  case _: B => // warn: unreachable
    |       ^^^^
    |       Unreachable case
 -- [E030] Match case Unreachable Warning: tests/warn/redundant-null.scala:24:7 -----------------------------------------
-24 |  case _ => // warn
+24 |  case _ => // warn: unreachable
    |       ^
    |       Unreachable case
 -- [E030] Match case Unreachable Warning: tests/warn/redundant-null.scala:25:7 -----------------------------------------
-25 |  case null => // warn
+25 |  case null => // warn: unreachable
    |       ^^^^
    |       Unreachable case
 -- [E121] Pattern Match Warning: tests/warn/redundant-null.scala:30:7 --------------------------------------------------
-30 |  case _ => // warn
+30 |  case _ => // warn: null only
    |       ^
    |       Unreachable case except for null (if this is intentional, consider writing case null => instead).
 -- [E030] Match case Unreachable Warning: tests/warn/redundant-null.scala:31:7 -----------------------------------------
-31 |  case null => // warn
+31 |  case null => // warn: unreachable
    |       ^^^^
    |       Unreachable case
 -- [E030] Match case Unreachable Warning: tests/warn/redundant-null.scala:32:7 -----------------------------------------
-32 |  case _ => // warn
+32 |  case _ => // warn: unreachable
    |       ^
    |       Unreachable case
 -- [E030] Match case Unreachable Warning: tests/warn/redundant-null.scala:33:7 -----------------------------------------
-33 |  case _ => // warn
+33 |  case _ => // warn: unreachable
    |       ^
    |       Unreachable case
 -- [E030] Match case Unreachable Warning: tests/warn/redundant-null.scala:37:7 -----------------------------------------
-37 |  case _ => println("unreachable") // warn
+37 |  case _ => // warn: unreachable
    |       ^
    |       Unreachable case
 -- [E030] Match case Unreachable Warning: tests/warn/redundant-null.scala:41:7 -----------------------------------------
-41 |  case _ => // warn
+41 |  case _ => // warn: unreachable
    |       ^
    |       Unreachable case
+-- [E121] Pattern Match Warning: tests/warn/redundant-null.scala:45:7 --------------------------------------------------
+45 |  case _ => // warn: null only
+   |       ^
+   |       Unreachable case except for null (if this is intentional, consider writing case null => instead).

tests/warn/redundant-null.scala

@@ -7,35 +7,39 @@ val n = null
 def f(s: A) = s match
   case _: n.type =>
   case _: A =>
-  case _: n.type => // warn
+  case _: n.type => // warn: unreachable
   case null =>
-  case _ => // warn
-  case _ => // warn
+  case _ => // warn: unreachable
+  case _ => // warn: unreachable
 
 def f2(s: A | B | C) = s match
   case _: A => 0
   case _: C | null | _: B => 1
-  case _ => 3 // warn
+  case _ => 3 // warn: unreachable
 
 def f3(s: A | B) = s match
   case _: A =>
   case _ =>
-  case _: B => // warn
-  case _ => // warn
-  case null => // warn
+  case _: B => // warn: unreachable
+  case _ => // warn: unreachable
+  case null => // warn: unreachable
 
 def f4(s: String | Int) = s match
   case _: Int =>
   case _: String =>
-  case _ => // warn
-  case null => // warn
-  case _ => // warn
-  case _ => // warn
+  case _ => // warn: null only
+  case null => // warn: unreachable
+  case _ => // warn: unreachable
+  case _ => // warn: unreachable
 
 def f5(x: String) = x match
-  case x => println("catch all")
-  case _ => println("unreachable") // warn
+  case x =>
+  case _ => // warn: unreachable
 
 def test(s: String | Null) = s match    
   case ss => 
-  case _ => // warn
+  case _ => // warn: unreachable
+
+def test2(s: String | Null) = s match    
+  case ss: String => 
+  case _ => // warn: null only